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Versions: 00 01 02 03 04 rfc2502                                        
INTERNET DRAFT                                       J.M.Pullen
Expires in six months                                  George Mason U.
                                                       U.of Central Florida
                                                     7 February 1998

    Limitations of Internet Protocol Suite for Distributed Simulation
                 in the Large Multicast Environment


Status of this Memo

     This document is an Internet-Draft.  Internet-Drafts are working
     documents of the Internet Engineering Task Force (IETF), its
     areas, and its working groups.  Note that other groups may also
     distribute working documents as Internet-Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other
     documents at any time.  It is inappropriate to use Internet-
     Drafts as reference material or to cite them other than as
     ``work in progress.''

     To learn the current status of any Internet-Draft, please check
     the ``1id-abstracts.txt'' listing contained in the Internet-
     Drafts Shadow Directories on ftp.is.co.za (Africa),
     nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
     ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast).


The Large-Scale Multicast Applications (LSMA) working group was chartered to
produce Internet-Drafts aimed at a consensus-based development of the
Internet protocols to support large scale multicast applications including
real-time distributed simulation.  This draft defines aspects of the Internet
protocols that LSMA has found to need further development in order to meet
that goal.

1.  The Large Multicast Environment

The Large-Scale Multicast Applications working group (LSMA) was formed
to create a consensus-based requirement for Internet Protocols to support
Distributed Interactive Simulation (DIS) [DIS94], its successor the High
Level Architecture for simulation (HLA) [DMSO96], and related applications.
The applications are characterized by the need to distribute a real-time
application over a shared wide-area network in a scalable manner such that
numbers of hosts from a few to tens of thousands are able to interchange
state data with sufficient reliability and timeliness to sustain a three-
dimensional virtual, visual environment containing large numbers of moving
objects.  The network supporting such an system necessarily will be capable
of multicast [IEEE95a,IEEE95b].

Distributed Interactive Simulation is the name of a family of protocols
used to exchange information about a virtual environment among hosts in
a distributed system that are simulating the behavior of objects in that
environment.  The objects are capable of physical interactions and can
sense each other by visual and other means (infrared, etc.).  DIS was
developed by the U.S. Department of Defense (DoD) to implement system for
military training, rehearsal, and other purposes. More information on DIS
can be found in [SSM96].

The feature of distributed simulation that drives network requirements is
that it is intended to work with output to and input from humans across
distributed simulators in real time. This places tight limits on latency
between hosts.  It also means that any practical network will require
multicasting to implement the required distribution of all data to all
participating simulators.  Large distributed simulation configurations are
expected to group hosts on multicast groups based on sharing the same
sensor inputs in the virtual environment.  This can mean a need for
hundreds of multicast groups where objects may move between groups in
large numbers at high rates.  The overall total data rate (the sum of all
multicast groups) is bounded, but the required data rate in any particular
group cannot be predicted, and may change quite rapidly during the

DIS real time flow consists of packets of length around 2000 bits at
rates from .2 per second per simulator to 15 per second per simulator.
This information is intentionally redundant and is normally transmitted
with a best-effort transport protocol (UDP), and in some cases also is
compressed.  Required accuracy both of latency and of physical simulation
varies with the intended purpose but generally must be at least sufficient
to satisfy human perception.  For example in tightly coupled simulations
such as high performance aircraft maximum acceptable latency is 100
milliseconds between any two hosts.  At relatively rare intervals events
(e.g. collisions) may occur which require reliable transmission of some
data, on a unicast basis, to any other host in the system.

The U.S. DoD has a goal to build distributed simulation systems with up to
100,000 simulated objects, many of them computer-generated forces that run
with minimal human intervention, acting as opposing force or simulating
friendly forces that are not available to participate.  DoD would like to
carry out such simulations using a shared WAN.  Beyond DoD many people
see a likelihood that distributed simulation capabilities may be
commercialized as entertainment.  The scope of such an entertainment
system is hard to predict but conceivably could be larger than the DoD
goal of 100,000.

The High Level Architecture (HLA) is a development beyond DIS that aims
at bringing DIS and other forms of distributed simulation into a unifying
system paradigm. From a distributed systems standpoint HLA is considerably
more sophisticated than DIS. For example attributes of distributed objects
may be controlled by different simulators.  From the standpoint of the
supporting network the primary difference between HLA and DIS is that HLA
does not call for redundant transmission of object attributes; instead it
specifies a "Run Time Infrastructure" (RTI) that is responsible to transmit
data reliably, and may choose to do so by various means including redundant
transmission using best-effort protocols. It is reasonable to say that any
network that can meet the needs of DIS can support HLA by DIS-like redundant
transmission, however this approach ignores the possibility that under HLA
some mixture of redundant and reliable transmission can make significantly
better use of network resources than is possible using DIS.

2.  Distributed Simulation (DIS and HLA) network requirements.

a.  real-time packet delivery, with low packet loss (less than 2%),
predictable latency on the order of a few hundred milliseconds, after
buffering to account for jitter (variation of latency) such that less
than 2% of packets fail to arrive within the specified latency, in
a shared network

b.  multicasting with thousands of multicast groups that can sustain
host group join in less than one second at rates of hundreds of joins
per second, (leave need not be so rapid but must also be fast because
holding groups open may delay opening other groups)

c.  multicasting using a many-to-many paradigm in which 90% or more
of the group members act as receivers and senders within any given
multicast group

d.  support for resource reservation; because of the impracticality
of over-provisioning the WAN and the LAN for large distributed
simulations, it is important to be able to reserve an overall capacity
that can be dynamically allocated among the multicast groups

e.  support for a mixture of best-effort and reliable low-latency
multicast, where best-effort predominates in the mixture

f.  support for secure networking, needed for classified military

3.  Internet Protocol Suite facilities needed and not yet available
for large-scale distributed simulation in shared networks.  These derive
from the need for real-time multicast with established quality of service
in a shared network.  (Implementation questions are not included in this
discussion.  For example, it is not clear that implementations of IP
multicast exist that will support the required scale of multicast group
changes for LSMA, but this appears to be a question of implementation,
not a limitation of IP multicast.)

3.1  Large-scale resource reservation in shared networks

The Resource reSerVation Protocol (RSVP) is aimed at providing setup
and flow-based information for managing information flows at pre-
committed performance levels.  This capability is generally seen as
needed in real-time systems such as the HLA RTI.  While RSVP has not
been deployed on the scale of the LSMA, its architecture does not
appear to pose any barriers to scaling to that level.  However, the
current RSVP draft standard does not support aggregation of
reservation resources for groups of flows, nor does it support
highly dynamic flow control changes.

Further, RSVP provides support only for communicating specifications
of the required information flows between simulators and the network,
and within the network.  Distributing routing information among the
routers within the network is a different function altogether,
performed by routing protocols such as Multicast Open Shortest
Path First (MOSPF). In order to provide effective resource reservation
in a large shared network function, it may be necessary to have a
routing protocol that determines paths through the network within the
context of a quality of service requirement.  An example is the
proposed Quality Of Service Path First (QOSPF) routing protocol
[ZSSC97]. Unfortunately the requirement for resource-sensitive routing
will be difficult to define before LSMA networks are deployed with RSVP.

3.2  IP multicast that is capable of taking advantage of all common
link layer protocols (in particular, ATM)

Multicast takes advantage of the efficiency obtained when the network
can recognize and replicate information packets that are destined to a
group of locations. Under these circumstances, the network can take on
the job of providing duplicate copies to all destinations, thereby
greatly reducing the amount of information flowing into and through
the network.

When IP multicast operates over Ethernet in a LAN and all subnets are
interconnected using the Internet Group Management Protocol (IGMP),
this is exactly what happens.  However, with the new high performance
wide-area technology Asynchronous Transfer Mode (ATM), the ability to
take advantage of data link layer multicast capability is not yet
available beyond a single Logical IP Subnet (LIS).  This appears to be
due to the fact that (1) the switching models of IP and ATM are
sufficiently different that this capability will require a rather
complex solution, and (2) there has been no clear application
requirement for IP multicast over ATM multicast that provides for
packet replication across multiple LIS.  Distributed simulation is
an application with such a requirement.

3.3  Hybrid transmission of best-effort and reliable multicast

In general the Internet protocol suite uses the Transmission Control
Protocol (TCP) for reliable end-to-end transport, and the User
Datagram Protocol (UDP) for best-effort end-to-end transport,
including all multicast transport services.  The design of TCP
is only capable of unicast transmission.

Recently the IETF has seen proposals for several reliable multicast
transport protocols (see [Mont97] for a summary). A general issue
with reliable transport for multicast is the congestion problem
associated with delivery acknowledgments, which has made real-time
reliable multicast transport infeasible to date.  Of the roughly 15
attempts to develop a reliable multicast transport, all have shown
to have some problem relating to positive receipt acknowledgments
(ACK) or negative acknowledgments (NAK). In any event, its seems
clear that there is not likely to be a single solution for reliable
multicast, but rather a number of solutions tailored to different
application domains. Approaches involving distributed logging seem
to hold particular promise for the distributed simulation application.

In the DIS/HLA environment, five different transmission needs can
be identified:
(1) best-effort low-latency multicast of object attributes that often
change continuously, for example position of mobile objects;
(2) low-latency reliable multicast of object attributes that do
not change continuously but may change at arbitrary times during the
simulation, for example object appearance (An important characteristic
of this category is that only the latest value of any attribute is
(3) low-latency, reliable unicast of occasional data among arbitrary
members of the multicast group (This form of transmission was
specified for DIS "collisions"; it is not in the current HLA
specification but might profitably be included there. The requirement
is for occasional transaction-like exchange of data between two
arbitrary hosts in the multicast group, with a low latency that makes
TCP connection impractical.);
(4) reliable but not necessarily real-time multicast distribution
of supporting bulk data such as terrain databases and object
enumerations; and
(5) reliable unicast of control information between individual
RTI components (this requirement is met by TCP).

All of these transmissions take place within the same large-scale
multicasting environment. The value of integrating categories (1)
and (2) into a single selectively reliable protocol was proposed by Cohen
[Cohe94].  Pullen and Laviano implemented this concept [PuLa95] and
demonstrated it within the HLA framework [PLM97] as the Selectively
Reliable Tranmission Protocol (SRTP) for categories (1) through (3).
Category (4) could be supported by a reliable multicast protocol such
as the commercial multicast FTP offering from Starburst [MRTW97], however
adequate congestion control has not been demonstrated in any such protocol.
There has been some discussion of using the Real-Time Streaming Protocol,
RTSP, for this purpose, however as the databases must be transmitted
reliably and RTSP uses a best-effort model, it does not appear to be

In summary, it is clear that a hybrid of best-effort and reliable
multicast (not necessarily all in the same protocol) is needed to
support DIS and HLA, and that the low-latency, reliable part of this
hybrid is not available in the Internet protocol suite.

3.4  Network management for distributed simulation systems

Coordinated, integrated network management is one of the more
difficult aspects of a large distributed simulation exercise.  The
network management techniques that have been used successfully to
support the growth of the Internet for the past several years could
be expanded to fill this need.  The technique is based on a
primitive called a Management Information Base (MIB) being polled
periodically at very low data rates.  The receiver of the poll is
called an Agent and is collocated with the remote process being
monitored. The agent is simple so as to not absorb very many resources.
The requesting process is called a Manager, and is typically located
elsewhere on a separate workstation.  The Manager communicates to all of
the agents in a given domain using the Simple Network Management Protocol
(SNMP). It appears that SNMP is well adapted to the purpose of distributed
simulation management, in addition to managing the underlying simulation
network resources.  Creating a standard distributed simulation MIB format
would make it possible for the simulation community to make use of the
collection of powerful, off-the-shelf network management tools that have
been created around SNMP.

3.5  A session protocol to start, pause, and stop a distributed
simulation exercise

Coordinating start, stop, and pause of large distributed exercises
is a complex and difficult task.  The Session Initiation Protocol
(SIP) recently proposed by the Multiparty MUltimedia Session Control
(MMUSIC) working group serves a similar purpose for managing large
scale multimedia conferences. As proposed, SIP appears to offer
sufficient extensibility to be used for exercise session control,
if standardized by the IETF.

3.6  An integrated security architecture

It appears that this requirement will be met by IPv6 deployment. A
shortcoming of the current Internet Protocol (IPv4) implementation
is the lack of integrated security. The new IPv6 protocol requires
implementers to follow an integrated security architecture that
provides the required integrity, authenticity, and confidentiality
for use of the Internet by communities with stringent security
demands, such as the financial community.  The possibility
that the IPv6 security architecture may meet military needs,
when combined either with military cryptography or government-
certified commercial cryptography, merits further study.

3.7  Low-latency multicast naming service

Name-to-address mapping in the Internet is performed by the Domain
Name Service (DNS).  DNS has a distributed architecture tuned to
the needs of unicast networking with reliable transmission (TCP)
that typically has latency of at least several seconds.  The
requirement of distributed simulation for agile movement among
multicast groups implies a need for name-to-multicast-address
mapping with latency of under one second for the name resolution
and group join combined.  This problem has been circumvented in
military simulations by using group IP addresses rather than names.
However, if distributed simulation is to grow into the domain of
commercial entertainment, a parallel to the need for DNS will arise.
Thus a low-latency naming service will be required.

3.8  Inter-Domain Multicast Routing for LSMA

While military LSMAs typically take place within a single
administrative domain, future entertainment LSMAs can be expected
to involve heavy inter-domain multicast traffic.  Standardized
protocols able to support large numbers of multicast flows across
domain boundaries will be needed for this purpose.

4.  References

[Cohe94]  Cohen, D., "Back to Basics", Proceedings of the 11th
          Workshop on Standards for Distributed Interactive
          Simulation, Orlando, FL, September 1994

[DIS94]   DIS Steering Committee, "The DIS Vision", Institute for
          Simulation and Training, University of Central Florida,
          May 1994

[DMSO96]  Defense Modeling and Simulation Office, High Level
          Architecture Rules Version 1.0, U.S. Department of
          Defense, August 1996

[IEEE95a] IEEE 1278.1-1995, Standard for Distributed Interactive
          Simulation - Application Protocols

[IEEE95b] IEEE 1278.2-1995, Standard for Distributed Interactive
          Simulation - Communication services and Profiles

[MRTW97]  Miller, K, K. Robertson, A. Tweedly, and M. White,
          "StarBurst Multicast File Transfer Protocol (MFTP)
          Specification", draft-miller-mftp-spec-02.txt, work in
          progress, January 1997

[Mont97]  Montgomery, T., Reliable Multicast Links webpage,

[PuLa95]  Pullen, J. and V. Laviano, "A Selectively Reliable Transport
          Protocol for Distributed Interactive Simulation", Proceedings
          of the 13th Workshop on Standards for Distributed Interactive
          Simulation, Orlando, FL, September 1995

[PLM97]   Pullen, J., V. Laviano and M. Moreau, "Creating A Light-Weight
          RTI As An Evolution Of Dual-Mode Multicast Using Selectively
          Reliable Transmission", Proceedings of the Second Simulation
          Interoperability Workshop, Orlando, FL, September 1997]

[SPW94]   Symington, S., J.M.Pullen and D. Wood, "Modeling and
          Simulation Requirements for IPng", RFC1667, August 1994

[SSM96]   Seidensticker, S., W. Smith and M. Myjak, "Scenarios and
          Appropriate Protocols for Distributed Interactive Simulation",
          draft-ietf-lsma-scenarios-00.txt, work in progress 13 June
          1996 (companion Internet Draft)

[ZSSC97]  Zhang, Z., C. Sanchez, W. Salkewicz, and E. Crawley, "Quality
          of Service Path First Routing Protocol",
          draft-zhang-qos-ospf-01.txt, work in progress September 1997

5.  Authors' Addresses

  J. Mark Pullen
  Computer Science/C3I Center
  MS 4A5
  George Mason University
  Fairfax, VA 22032

  Michael Myjak
  Institute for Simulation and Training
  University of Central Florida
  Orlando, FL 32816

  Christina Bouwens
  ASSET Group, SAIC Inc.
  Orlando FL

Expiration: 7 August 1998